Electron emitter



June 10, 1952 JACOBS ET AL 2,600,112

ELECTRON EMITTER Filed June 30, 1948 1N1 'ENTOR. Harold Jacobs 1 $50265 1 E5 I M? M Patented June 10, 1 952 ELECTRON ElVIITTER Harold Jacobs, St. Albans, and George Bees, Woodside, N. Y.,"assignors to SylvaniaElectric Products Inc.,'a corporation of Massachusetts Application June so, 1948, Serial No. 36,138

l2 Claims.

This invention relates toelectfdn'mitteisand tothe method o'f'their preparaticnfouusein electron discharge devices.

Althoughthere has been some indication in the literature that it was recognizedthat alkali metal oxides could be vaporized onto electrodes to obtaina thin monatomic layer which would possess good secondary emission, infact, superior'to that obtained with 'monatoi'nic layers of the alkali metals themselves, no process suitablev for commrcial adaptation had previously been devised which would'per mit of the preparation of these materials in this manner. In contrast to the alkali earth metals, compounds of the alkali metals such as cesium carbonate are very deliouesceut and will not remain in the solid state fora suillcient length of time to permit of their being treated by a vaporization process, that is in anymanner which is commercially feasible.

For this reason commercial operations have heretofore been limited to the use of the alkali metals as such. When these areused it is well known in manufacturing circles that several cycles of "sparking, rebaking and reflashing have to be performed to 'get the right balance between electrical. leakage and electron emission. The electrical leakage is almost always high and often so high as to interfere with electric current measurements. {This results in one of theprimary causes of noise and dark current.

The object of this invention is to provide a commercially satisfactory method of making a secondary electron emitter having a layer of an alkali metal compound. Another object of this invention is to provide a commercially satisfactory method of applying cesium'oxide to electrode materials by a vaporization technique.

A still further objector this invention is to improve the quality of such electrodes.

It has been found that these objectives and other advantages can be obtained by operating in accordance with the teachings of this invention. In accordance therewith, it has been found that suitable secondary electron emitters can be prepared by a commercially satisfactory process when the alkali metal compound such as CS2CO: is suspended in a mixture of ethylcellulose and amyl acetate and then either cataphoretically deposited or 'sprayedonto a support from which it is subsequently vaporized by means of heat and deposited upon the electrode surface.

although the techniques and teachings of this invention can be applied -to "the use of the alkali earth metal compOundsLthe problmsbftlieliiibr art were greatest with respect to'the'alliali metal compounds. It is, therefore, particularlygr'atifying that compoundsof csium-whichinaddition to having the highestelectron emissivity of the alkali metals, is the most active of all inetals and also the most electropositive, can readilybe applied by the method of this invention. further gratifying in that cesium, has a repute tion for beingone of the mostdifiicult-metals to handle particularly on-a commercial scalel where it is economically unsound practice to have to resort to sparking and baking, teohniques and where leakage problems cannot be tolerated. Since cesiurnhas been recognized as being tone of the most difiicult of the metals to handle, itwill readily be apparent that whenever cesium is given as an example in this speciflcationany of the other alkali metals as well as the alkali earth metals could besubstitutedtherefor. v

The most satisfactory compounds arethose which can be activated by the heat treatment,-- for example, the carbonates, fiuoride rand chlorides (i. e. CsF, BaF-z, CS2CO3, ;Ba iz, aco3 ;The carbonates readilylend themselves to decomposition by heat treatment to form the oxides. 'Ifhe oxides in turn by further heating become activated. I A

Silver compounds have been found'to be desirable addition agents which tend to enhance the emission characteristics of the electrode. These compounds can be incorporated and mixed with the alkali metal compounds as-they are putiin suspension. For this reason the preferred examples of suitable suspensions given, hereinafter include silver oxide as one of the eomporifits.

An electronic tube which was manufactured in accordance with a 'preferrdembofiimjiit of this invention is illustrated in the drawings in which: g V

Fig. l is a longitudinal section. showing the arrangement of parts, 4 v a Fig. 2 is a plane view "along "the linerint Fig. land m, ...s

Fig. 3 is 'an'enlarg'earrag emery viewoftl'ie electrodes'and grid'showh iii Eig'l. Figure 4 is asectional elvationof theele'etrode shownin Figure l. V y

This tube contains 'afilament (0 mounted? a mica disc u. tungsten.ioogmesngnu'ljlt a inch diameter hole in the grid suppor It with the grid being positioned 'betwee'n"the filament In and an electrode IBgthe suffabebf Ithe electrode l8 being coated with vfirst 'ajbamer material 20 andthen with a layer of'aii *allcali metal oxide or alkali metal compound 2|. A getter 22 positioned near the top of the tube envelope 24 was used in the usual manner to clean up the residual gases after the degassing operation.

In order to overcome the deliquescent nature of the alkali metals and their compounds, and still make feasible the vaporization of such alkali compounds from a support onto a desired electrode surface, the alkali metal is preferably suspended in a solution of ethylcellulose in amyl acetate. It has been found that if this is done, alkali metal compounds including those of cesium after being milled, can be permitted to stand for weeks and after rolling for one hour still retain the nature and consistency suitable or spraying or electrophoretic deposition.

A preferred example of a spray suspension which has consistently been used with success is made up as follows:

Cesium carbonate-4 parts by weight Silver oxide1 part by weight Amyl acetate-5 parts by volume Ethylcellulose4.6 parts by weight in amyl acetate5 parts by volume Milling time is approximately 24 hours.

In accordance with another preferred embodiment of the spray solution of this invention, 50 cc. of 4.6 percent ethylcellulose in amyl acetate is added to 50 grams of 80% cesium carbonate and 20% silver oxide whereupon a further addition of 50 cc. of amyl acetate can be added.

The total mixture is then milled for a period of 48 hours.

This mixture upon being sprayed on the surface appears to be stable in air and can readily be used as a source material for the vaporization of the alkali metal oxides, or other alkali metal compounds.

In those instances where it has been found to be advantageous to have the spray coating abrasion resistant as well as stable it has been found to be advantageous to bake the coating at 400 to 500 C. The resulting coating is hard, dry and capable of withstanding considerable abrasion and can subsequently be broken down to form the activated material.

The stable surface obtained either directly after spraying or after the baking operation can now be used for the vaporization of a thin coating onto the secondary emission electrode or 'cLvnode. This can be accomplished in many ways. Preferably this is done by placing the material with its support within the tube during its fabrication, degassing the tube in the normal manner and then subsequently bombarding the coated area in order to raise the temperature to the point at which the alkali metal carbonate or other compound breaks down and vaporizes.

In the case of the carbonate, the vapor is in the form of the oxide which is given off and is deposited in a thin film on the other surfaces within the tube proper. Although this vaporization. will cause the oxides to deposit on surfaces other than the dynode no electrical leakage will take place in view of the non-conductivity of the oxide as compared with that of the metal. Furthermore, when the other parts of the tube become heated they will, in turn, give off the oxide which will redeposit itself on cool surfaces within the tube which will include the dynode surface if proper precautions are taken during the aging in of the tube itself.

In accordance with still another method. the

support containing the alkali metal compound to be vaporized can be placed near a filament which can be heated to give oif sufficient heat to break down the alkali metal compound and cause it to vaporize. During the course of the work done in line with this invention, a tungsten filament was placed close to a nickel sleeve coated with an oxide of cesium whereupon the tungsten was kept at a yellow heat at the same time that the nickel sleeve was heated and cesium oxide evaporated on the tungsten. The emission from this filament at 775 C. was about .5 amp. per centimeter squared although the filament was heated to 1500 C. for brightness as long as one hour, its activity was not destroyed. Furthermore no leakage was observed.

In accordance with another preferred embodiment of this invention, the sensitizing material can be coated on the back of the getter so that during the getter flash the alkali metal compound is volatilized and deposited on the surface of the dynode. A cesium carbonate silver oxide surface is preferred. When this is done, the following exhaust procedures are preferably used.

First the tube is baked out at 400 C. for about hour. Following this the grid is set at 60 volts and the filaments kept at a red hot heat. At the same time the getter is quickly degassed and flashed while vaporizing and spraying a mixture of cesium oxide, silver oxide around the base of the tube. When this procedure is followed by a bombardment of the No. 1 grid, any cesium compound which has been deposited thereon will be volatilized therefrom some of which will then re-deposit onto the dynode which is closely spaced therefrom, thus increasing its secondary emission properties.

The composition of the material upon which the secondary emission material is deposited, also plays an important part in determining the secondary emission characteristics of the elec-' trode. In accordance with this invention, it has been found that inert materials having a low heat of formation in the oxide form such as rhodium, platinum, gold, silver, copper appear to give the best results when the secondary emission surface is to be operated at a relatively cool temperature. However, in those cases in which the secondary emission surface is to be operated at a high temperature those materials whose oxides are somewhat conductive and have a high heat of formation are possible. Such materials include the nickel iron chromium alloys and aluminum.

Furthermore, it is preferable to keep the dynode upon which the secondary emission material is to be deposited, at a negative potential during the evaporation of the alkali metal compound. This might be explained theoretically in that the keeping of a negative field potential on a dynode during the evaporation prevents electron bombardment of the dynode durin this period and thus provides a cooler surface for the condensation to take place.

Furthermore, if any ions are formed in the intervening space, the negative potential will most probably produce the ions, and the cesium or other alkali metals being vaporized, will deposit more accurately on the dynode surface. In actual practice, it has been found that flashing in this manner has enhanced the secondary emission from initial values of three up to nine.

Not only has the process of this invention ity to oxide coated cathodes.

enabled us to successfully vaporize the alkali metal compounds in a manner which is commercially feasible but it has enabled us to provide secondary emission surfaces which would hold up and give satisfactory long life in those cases when these surfaces are used in the close vicin- Normally when secondary emission surfaces are used in such manner, the oxide coated cathode must be shielded from the secondary emission surface. However, surfaces prepared in accordance with this invention, have been used successfully-within a few thousandths of an inch of oxide coated cathodes while maintaining life properties for over 200 hours.

While the above description and drawings submitted herewith disclose preferred and practical embodiments of the secondary emission electrodes and their method of manufacture, it will be understood by those skilled in the art, that the specific details of manufacture as shown and described are by way of illustration and are not to be construed as limiting the scope of the invention.

What is claimed is:

l. The method of preparing secondary electron emittin electrodes comprising coating a support with an alkali metal compound suspended in ethyl cellulose. introducing said coated support into a tube and placin it adjacent to an electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of the alkali metal compound by vaporizing the alkali metal compound from said support onto said electrode while holding said electrode at a negative potential.

2. The method of preparing secondary electron emitting electrodes comprising coating a support with an alkali metal carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, evacuating said tube,

and forming a secondary electron emitter having a thin monatomic layer of an alkali metal oxide by vaporizing an alkali metal oxide from said support onto said electrode while holding said electrode at a negative potential.

3. The method of preparing secondary electron emittin electrodes comprising coating a support with an alkali metal fluoride suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of an alkali metal fluoride by vaporizing the alkali metal fluoride from said support onto said electrode while holding said electrode at a negative potential.

4. The method of preparing secondary electron emitting electrodes comprising coating a support with cesium carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of the cesium oxide by vaporizing the cesium oxide from said support onto said electrode while holding said electrode at a negative potential.

5. The method of preparing secondary electron emittin electrodes comprising coating a support with an alkali metal compound suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an inert metal coated electrode oi said tube, evacuating said tube, and forming a secondary electron emitter having a, thin monatomic layer of the alkali metal compound by vaporizing'the alkali metal compound from said support onto said inert metal coated electrode while holding said electrode at a negative potential.

6. The method or" preparing secondary electron emitting electrodes comprising coating 21 support with cesium carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it, adjacent to an-electrode of said tube, said electrode being coated with an inert metal, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of cesium oxide by vaporizing cesium oxide from said support onto said precoated electrode while holding said electrode at a negative potential.

7. The method of preparing secondary electron emitting electrodes comprising coating a support with an alkali metal carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, said electrode having a coating of an inert metal, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of alkali metal oxide by vaporizing the alkali metal oxide from said support onto said electrode while holding said electrode 7 at a negative potential.

8. The method of preparing secondary electron emitting electrodes comprising coating a support with barium carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of barium oxide by vaporizing the barium oxide from said support onto said electrode while holding said electrode at a negative potential.

9. The method of preparing secondary electron emitting electrodes comprising coating a support with an alkali metal carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to a copper coated electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of alkali metal oxide by vaporizing the alkali metal oxide from said support onto said electrode while holding said electrode at a negative potential.

10. The method of preparing secondary electron emitting electrodes comprising coating a support with cesium carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to a copper coated electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of cesium oxide by vaporizing the cesium oxide from said support onto said electrode while holding said electrode at a negative potential.

11. The method of preparing secondary electron emitting electrodes comprising coating a support with an alkali metal carbonate suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to a nickel chromium iron alloy electrode of said tube, evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of the alkali metal carbonate by vaporizing the alkali metal carbonate from said support onto said electrode while holding said electrode at a negative potential.

12. The method of preparing secondary electron emitting electrodes comprising coating a support with cesium carbonate and silver oxide suspended in ethyl cellulose, introducing said coated support into a tube and placing it adjacent to an electrode of said tube, said electrode having a coating of an inert material selected from the group consisting of rhodium, silver, platinum, gold, copper and nickel chromium alloys evacuating said tube, and forming a secondary electron emitter having a thin monatomic layer of cesium and'silver oxide by vaporizing the cesium and silver oxide from said support onto said electrode while holding said electrode at a negative potential.

HAROLD JACOBS. GEORGE HEES.

8 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,464,124 Wilson Aug. 7, 1923 1,842,161 Fredenburgh June 19, 1932 1,935,939 Case Nov. 21, 1933 1,966,220 Rentschler et al July 10, 1934 2,003,344 DeBoer et a1 June 4, 1935 2,187,234 Gorlich Jan. 16, 1940 2,198,329 Bruning Apr. 23,- 1940 2,317,754 Gorlich Apr. 27, 1943 2,404,343 Henderson et al July 16, 1946 

1. THE METHOD OF PREPARING SECONDARY ELECTRON EMITTING ELECTRODES COMPRISING COATING A SUPPORT WITH AN ALKALI METAL COMPOUND SUSPENDED IN ETHYL CELLULOSE, INTRODUCING SAID COATED SUPPORT INTO A TUBE AND PLACING IT ADJACENT TO AN ELECTRODE OF SAID TUBE, EVACUATING SAID TUBE, AND FORMING A SECONDARY ELECTRON EMITTER HAVING A THIN MONATOMIC LAYER OF THE ALKALI METAL COMPOUND BY VAPORIZING THE ALKALI METAL COMPOUND FROM SAID SUPPORT ONTO SAID ELECTRODE WHILE HOLDING SAID ELECTRODE AT A NEGATIVE POTENTIAL. 